1. Field of the Invention
This invention relates generally to bolometer type sensors for detecting thermal radiation and more particularly to ultra sensitive silicon bolometer type sensors usable for passive or active imaging at millimeter (MM) wavelengths.
2. Description of Related Art
Infrared (IR) bolometers are used and proposed for use in many new applications. The principal application is construction of thermal cameras. Interest in bolometers stems from the fact that their performance has significantly improved, they're sensitive at much longer wavelengths, and offer higher operating temperatures. Specifically, IR cameras, with large bolometer arrays have achieved a sensitivity, a Noise Equivalent Temperature resolution (NEΔT) better than ≈0.1K. Such performance is less than that of quantum detectors, however, for many applications it is adequate and cost effective. Improved bolometer performance is achieved primarily through improved thermal isolation, made possible with advances in IC micro-machining technology. The thermal isolation achieved is about an order of magnitude from radiation limited isolation.
Bolometers inherently operate at slower rates than quantum detectors. However, with staring focal plane arrays, the slow speed limitation is alleviated, since the pixel integration times correspond to the frame rate, and is much longer than line times in scanning systems. Thus the main obstacle to making bolometers more sensitive are practical limitations in thermally isolating each pixel element. With improved thermal isolation, the bolometers performance will directly improve and thereby find wider applications, including potential replacement for cryogenic FLIR cameras. With ideal thermal isolation, the anticipated NEΔT improvement is about an order of magnitude in sensitivity.
LWIR and MWIR silicon bolometers having a new operating mode are disclosed in U.S. Pat. No. 6,489,615 entitled “Ultra Sensitive Silicon Sensor”, issued to Nathan Bluzer, the present inventor, on Dec. 3, 2002. This patent is assigned to the present assignee and is incorporated herein by reference in its entirely.
In U.S. Pat. No. 6,489,615, electro-thermal feedback is utilized for removing thermal conductance between an absorber element of a bolometer pixel in a thermal radiation sensor assembly and the environments through its mechanical support structure and electrical interconnects, thereby limiting the thermal conductance primarily through photon radiation. Zeroing the thermal conductance associated with the mechanical support structure and electrical interconnects is achieved by electro-thermal feedback that adjust the temperature of an intermediate stage and the mechanical support structure as well as the electrical interconnects to equal the bolometer's absorber element temperature.